Prime Minister Theresa May has said she is "appalled" by the escalation of government bombing in the Syrian capital.
Intense shelling and air strikes on rebel-held Damascus suburbs have killed nearly 130 people since Sunday, marking the deadliest bombardment of the region in three years.
Mrs May, speaking during Prime Minister's Questions, called on President Bashar Assad to cease his "campaign of violence" and allow humanitarian aid in.
She also took aim at Assad's Russian backers and called on President Putin to "ensure this violence stops".
Suburbs targeted by the Syrian government - scattered across an area known as eastern Ghouta - have been subjected to a weeks-long bombardment that has killed and wounded hundreds of people.
Opposition activists said government forces have brought in more reinforcements in recent days, suggesting a major assault is imminent to recapture the last main rebel stronghold near Damascus.
SNP Westminster leader Ian Blackford, raising the issue at PMQs, said: "At least 194 people have been killed in the last 48 hours in eastern Ghouta.
"Can the Prime Minister tell the House what discussions her Government has had with UN colleagues since Sunday on enforcing existing UN resolution calling for an end to sieges of civilian areas and attacks on civilians?
Mrs May said: "We are appalled by the escalation of air strikes in eastern Ghouta and are deeply concerned by the reports that have been coming through of the ongoing deliberate targeting of civilians and civilian infrastructures.
"This is in blatant violation of international humanitarian and human rights law and we as the United Kingdom government certainly do call on the regime and its backers to cease this campaign of violence.
"They should respect international humanitarian law, protect civilians and allow rapid and unfettered humanitarian access."
She added: "We will be ensuring that we will be talking to our colleagues within the UN to ensure that the best possible approach can be taken in relation to these issues.
"It's not just about the Syrian government, it's about the backers of the Syrian government as well, and we call on all their backers including Russia to ensure that this violence stops and that those people who are in need of help are given that help."